1. Field of the Invention
This invention pertains to composites of a suitable germanate glass, particularly barium gallogermanate (BGG) glass, and spinel, and to a process for forming composites of the glass and the spinel by bonding them together.
2. Description of Related Art
There are numerous sensor and laser systems operating in the visible-infrared (IR) wavelength band of 0.4-5 μm which require windows, domes and other structures for protection. The size, shape and desired properties of the windows/domes depend on the application. These applications can be very military specific, such as sensor windows and domes on an aircraft; reconnaissance windows on manned and unmanned aerial vehicles and missile domes. On the other hand, the applications can be commercial, such as small and inexpensive lenses for infrared optics and windows for all visible and mid-wave lasers. Materials choice for any application is strongly dependent on the required properties, desired sizes and shape and material availability. For example, a dome shaped in a hemispherical or hyperhemispherical window might be required for missiles or some sensor applications.
Magnesium aluminate spinel, from here on is referred as spinel, is a candidate material for many of the applications in the 0.4-5 μm wavelength region. Spinel has a cubic crystal structure and transmits in the visible-IR region. It is a very hard material and consequently, its grinding and polishing costs are high. Applying a glass coating on the spinel substrate can significantly reduce its grinding and polishing costs. The index of refraction of glass and spinel can be matched to minimize reflection losses. The glass can be readily polished using traditional glass polishing techniques and therefore, considerably reduce the polishing costs of Spinel.
A new product is disclosed herein based on a BGG glass-spinel composite substrate. The new germanate glass is designed so that it can be easily bonded to a spinel substrate. The process for bonding the new glass to a spinel substrate is also disclosed. The bonded composite is very strong and withstands chemical and environmental abuse. The glass acts as an excellent dielectric layer in missile domes and window designs for applications requiring transmission in the visible, IR and microwave frequencies. For example, the composite can contain an electromagnetic interference (EMI) grid.
Many of the barium gallogermanate glasses disclosed in the U.S. Pat. No. 5,305,414 cannot be used here because of the large coefficient of thermal expansion (CTE) mismatch with spinel. Due to this large CTE mismatch, the BGG glass from previous inventions delaminates from spinel substrates during thermal cycling. The BGG glass suitable herein bonds very well to spinel and does not delaminate.